Lignocellulosic Biomass Processing - Food and Industrial Bioproducts and Bioprocessing - page 296

aromatic carbonyl compounds are partially responsible for the odor of bio-oil and the
reduction of aldehydes to alcohols will reduce the odor of bio-oils. Aldehydes and ketones
can be hydrogenated with a wide variety of catalysts.
Substituted phenols make up a larger portion of the bio-oils. Phenols from the bio-oils of
pine contain 3-methoxy groups along with a wide variety of other functional groups in the
1- position. Phenols and substituted phenols yield the corresponding cyclohexanes when
hydrogenated and the pungent odor associated with phenols will not be present after
hydrogenation. Hydrogenation of phenolic compounds will have the greatest impact on the
chemical and physical properties of bio-oils.
Only about one-third to one-half of the compounds in crude bio-oils can be analyzed by
gas chromatography/mass spectrometry (GC/MS). The non-volatile compounds are dimers,
trimers and oligimers of carbohydrate and lignin monomeric units. These high molecular
weight compounds will also be hydrogenated and with some of the large molecules will be
cleaved to monomeric units by removing the oxygen linkage between the monomeric units.
Catalytic HDO research has, to date, been based on application of traditional petroleum
catalysts. Moderate successes in applying these HDO catalysts on bio-oils have been
attained. However, specialized catalysts for bio-oil application, as opposed to petroleum
applications, have yet to be developed. The complex chemistry of bio-oils presents a special
challenge compared to that of processing crude petroleum oils that are mainly composed of
hydrocarbons. A promising approach identified by Bridgwater and Cottam (1992) is the
identification of a modified zeolite that is more selective toward bio-oil components and
desired products.
Zeolites are various types of crystalline aluminosilicates that occur in nature and can also
be synthesized (Augustine, 1996). More than 100 varieties of zeolites are currently available
with new candidate zeolites added monthly. Zeolites, such as ZSM-5, HZSM-5 and MCM-
41 in various forms, have been shown to be effective in catalyzing pyrolysis vapors via
catalytic cracking to produce liquid hydrocarbons suitable for fuels (Adam et al ., 2005 ;
Diebold and Scahill, 1988 ; Evans and Milne, 1988 ; Horne et al ., 1997 ; Sharma and Bakhshi,
1993 ; Renaud et al ., 1988). Catalytic cracking (as opposed to the milder HDO treatment) of
the liquid bio-oils themselves has also been performed with zeolites, both by treating
revaporized bio-oils (Renaud et al ., 1988) or by directly treating liquid bio-oils (Samolada
and Vasalos, 1997; Evans and Milne, 1988). Relatively low yields and rapid coking resulting
in deactivation of catalysts have both been reported as problems from the cracking of bio-oil
liquids or revaporized bio-oils.
HDO treatments of bio-oils with metallic catalysts adopted from the petroleum industry
have been performed (Baker and Elliott, 1988; Bridgwater and Cottam, 1992; Czernik et al .,
2002 ; Centeno et al ., 1995 , 1997 , 1999 ; Conti et al ., 1997 ; Ferrari et al ., 2002 ; Puente et al .,
1999 ; Oasmaa and Boocock, 1992 ; Zhang et al ., 2003). Researchers have shown that a two-
stage process is required (Baker and Elliott, 1988; Gagnon and Kaliaguine, 1988). The first
stage applies a mild hydrogenation at relatively low temperatures below about 270 °C. Full
HDO of bio-oil requires temperatures above 300 °C, which results in polymerization of highly
oxygenated components of raw bio-oils (Elliott and Neuenschwander, 1996). Therefore,
currently the HDO stage at higher temperatures must be reserved for application after the first
mild hydrogenation stage. HDO of bio-oils with zeolites has not been reported in the literature.
Zeolites are effective hydrogenating catalysts as well as proven cracking catalysts. However,
zeolites have to be tested as hydrogenation catalysts for bio-oils. The problems of low yields,
rapid coking and deactivation of catalysts reported for zeolytic cracking of bio-oils are not
expected to occur in the application of mild hydrogenation followed by HDO.